1. Field of the Invention
The present invention relates to an optical element provided with a conductive film having a subwavelength aperture and a surface topography formed therein, an optical head and optical data recording/reading apparatus using the optical element.
2. Description of the Related Art
An optical recording medium, such as a CD-ROM (Compact Disk-Read Only Memory) or DVD (Digital Video Disk), has various advantages including the capability of storing large quantities of data, compact design, portability, and robustness. As recording media and data recording/reading apparatuses drop in price, such an optical recording medium becomes increasingly attractive. The data density in optical recording media is desirably higher particularly for storing long-playing video data.
It is known well that the data density in optical recording media is determined by the size of pits on the optical recording medium, which is limited by the diameter of a focused laser beam on the optical medium. Accordingly, the data density in optical media can be increased by reducing the diameter of the laser beam.
In the case of an existing optical system in which a distance between an optical head and an optical recording medium is much greater than the wavelength of the laser beam, the minimum length of pits is determined by diffraction limit. More specifically, the diameter of a focused laser beam on the optical medium is limited to λ/2 by diffraction, where λ is the wavelength of the laser beam. This is a known phenomenon called the diffraction limit. For example, even when a blue laser is used which is a visible-light laser having the minimum wavelength at present, the minimum length of pits is limited to about 200 nm.
However, the length of pits can be reduced to 50 nm or less through near-field optics, in which an optical head having an aperture smaller than the wavelength of laser light is placed at a distance not larger than the size of the aperture from the optical recording medium and scanned over its surface. In this near-field optics, the length of pits is determined only by the size of the aperture. Accordingly, higher data densities and higher-speed data writing and reading can be achieved through the near-field optics.
These advantages of the near-field optics can be also obtained using a red diode laser, which is at present reliable and easily available at relatively low cost. In addition, it is possible to produce the optical head by directly coupling to an optical fiber or a semiconductor waveguide without the need of using a relatively heavy and big objective lens. This allows mechanical design of flying-type or contact-type head to be simplified.
A typical near-field scanning optical head is made by using a tapered optical fiber to achieve such an aperture smaller than the wavelength of laser light. However, the laser light is subject to substantial attenuation during propagation to the sub-wavelength aperture and thereby it is difficult to obtain a sufficient intensity of light required for writing data on the optical recording medium. See the following papers:    E. Betzig et al “Near-Field Optics: Microscopy, Spectroscopy, and Surface Modification Beyond the Diffraction Limit” (Science, Vol. 257, pp. 189–194, July 1992)    G. A. Valaskovic et al. “Parameter Control, Characterization, and Optimization in the Fabrication of Optical Fiber Near-Field Probes” (Applied Optics, Vol. 34, No. 7, pp. 1215–1227, 1995).
In a typical near-field optical head, material forming the sub-wavelength aperture is usually metal. However, the amount of light (or throughput) passing through the sub-wavelength aperture made of metal is very small: The throughput, which is defined to be IT/Iinc, where IT is the total transmitted power density or intensity at the aperture exit and Iinc is the total incident power intensity, is predicted to decrease with aperture diameter in approximate proportion to
                    I        t                    I        inc              ⁢                  ∼                  (                  d          λ                )            4        ,where d is the diameter of the aperture and λ is the wavelength of light (see H. A. Bethe, “Theory of Diffraction by Small Holes,” Physical Review, Vol. 66, Nos. 7 and 8, pp. 163–182, October 1944).
On the other hand, there is known a fact that the intensity of light passing through one or more subwavelength-diameter aperture perforating through a conductive film is remarkably increased by periodically arranging a plurality of apertures or providing a periodic surface topography associated with at least one aperture on the conductive film. See the following U.S. patents:    Ebbesen et al. U.S. Pat. No. 5,973,316;    Kim et al. U.S. Pat. No. 6,040,936;    Ebbesen et al. U.S. Pat. No. 6,052,238;    Ebbesen et al. U.S. Pat. No. 6,236,033 B1; and    Kim et al. U.S. Pat. No. 6,285,020 B1.
According to experimental results, the rate of increase in the intensity of light in some cases reaches up to IT/Iinc≈2. It is thought that the light incident on the surface of the conductive film interacts in a resonant way with a surface plasmon (SP) mode on the surface, resulting in enhanced electromagnetic fields at the conductive surface and thence enhanced transmission of light through at least one aperture in the conductive film.
An improved near-field optical head has been disclosed in Japanese Patent Application Unexamined Publication No. P2001-291265A, which corresponds to U.S. patent application Ser. No. 60/185,239 filed Feb. 28, 2000 and Ser. No. 09/721,694 filed Nov. 27, 2000, each of which are incorporated herein by reference. This conventional near-field optical head uses surface plasmon-enhancement caused by a subwavelength-diameter aperture and a periodic surface topography to achieve a very high power density of transmission of light and fine resolution. A schematic structure of this conventional near-field optical head is shown in FIG. 1.
In FIG. 1, a read/write head 500 is provided with a waveguide 510 and a plasmon-enhanced device (PED) 520. The waveguide 510 is tapered to make the cross-sectional area of an end surface 512 smaller, which is placed near an optical recording medium 50. A distance z between the end surface 512 and the surface of the optical recording medium 50 is set to the order of the diameter of an aperture 530.
The plasmon-enhanced device 520 is formed on the end surface 512 to enhance the transmission efficiency of light incident from the waveguide 510 and passing through the plasmon-enhancing device 520. The plasmon-enhanced [etc.] device 520 is provided with a metal film 522 that is preferably made of silver and has the aperture 530 perforating through the plasmon-enhancing device 520. The resolution of this read/write head 500 is determined by the dimensions of the-aperture 530. The diameter d of the aperture 530 is equal to or smaller than the wavelength of the incident light and corresponds to the dimensions of a pit recorded on the optical recording medium 50. The transmitted light has power intensity sufficient for forming pits. In the case of the optical recording medium 50 being of phase-change type, the intensity of writing light is set to be sufficiently high to the extent of locally fusing the phase-change optical recording medium.
The metal film 522 is further provided with a periodic surface topography 540, which allows a very large amount of transmitted light to form a subwavelength read/write light spot, resulting in a data density much higher than obtained by diffraction limit. A smaller spot causes both a higher data density and a higher reading speed. These advantages can be achieved by using a commercially available and not expensive laser, that is, without having to use a shorter-wavelength laser.
As described above, such enhanced transmission of light using the surface plasmon-enhancement needs the application of incident light to the surface topography formed around the small aperture. In the case of a single aperture having no surface topography, the incident light is simply applied to the small aperture.
Accordingly, in the case of using the surface plasmon-enhancement, the incident light should be applied to a wider area including the surface topography, which for fixed total laser power means that the power density of light is lower. Since the power density of light for writing information on the optical recording medium must be higher than a certain threshold, it is necessary to apply an increased power of incident light to the wider area so as to obtain a sufficient power density of light passing through the small aperture.
Further, the application of an increased power of incident light to the conductive film causes the conductive film and the surface of the optical recording medium adjacent thereto to increase in temperature, resulting in decreased reliability.